The staff of Clinical Studies Section in Boston, Massachusetts has sifted a large volume of clinical data to identify patients for etiologic studies. These unusual patients are usually found through attendance at clinical conferences and rounds, and through special referrals. With informed consent, epidemiologic studies are made of the environmental and genetic background and laboratory studies are performed to clarify biologic mechanisms of carcinogenesis. At present, several family aggregates of cancer are under study, e.g., Turcot's syndrome in two families; familial ureteral cancer, ataxia-pancytopenia syndrome, breast cancer-sarcoma syndrome. Also, a computerized Registry of Survivors of Childhood Cancer has been established through collaboratton of the Clinical Studies Section with the Sidney Farber Cancer Institute. The 700 registrants represent approximately 2% of all survivors of U.S. childhood cancer in the last two decades. Patients in the series are being studied for second malignancies, cancer in close relatives, and genetic effects in progeny.